Robert V. Gentry,1* Gary L. Glish,2 and Eddy H. McBay2

1Physics Department, Columbia Union College, Takoma Park, MD 20012

Abstract. A very sensitive helium leak detector
was utilized to measure the helium liberated from
groups of zircons extracted from six deep granite
cores. The observed low differential loss of
gaseous helium down to 2900 m (197°C) in these
ancient Precambrian rocks is easily attributable
to the greater diffusion of He at higher
temperatures rather than losses due to corrosion
of the zircons. This fact strongly suggests that
deep granite burial should be a very safe
corrosion-resistant containment procedure for
long-term waste encapsulation.

Recent mass spectrometric studies (Gentry, et
al. 1982) have revealed that lead has been retained
in zircons extracted from deep (960 m to
4310 m) granite cores where the ambient temperature
increases from 105°C to 313°C at the
greatest depth. As a follow-up to those experiments
we now report the results of differential
helium retention in similar zircons extracted
from the same granite core samples which were
used in the lead analyses (Laney and Laughlin,
1981).

The procedure for separating the zircons from
the six different granite cores (from depths of
960, 2170, 2900, 3502, 3930, and 4310 m) was the
same as that used in the previous experiments.
The high-density fractions, obtained by passing
the crushed core samples through different methylene iodide separating funnels, were thoroughly
washed with acetone before being placed on a
standard microscope slide. A fine-tipped needle
was used to pick out the individual zircons with
the aid of a polarizing microscope. Groups of
these separated zircons, usually about 10 in
number, were then loaded onto the platinum
filament of the thermal inlet probe of the mass
spectrometer for differential helium analysis.

The helium measurements were performed on a
Leybold-Heraeus model F helium leak detector that
had a Chemical Data Systems Pyrolysis unit
interfaced to the test port. The leak detector
has a detection limit of less than 10−10 cm3/sec
when operating in the dynamic mode. (The
instrument could have been operated in a near-static mode with increased sensitivity down to
~l0−11 cm3/sec of He, but our experiments did not
necessitate this increased sensitivity.)

In our initial series of measurements our
spectrometer was calibrated against a 5 (±0.5)
× 10−8 cm3/sec standard He leak. A subsequent
recalibration with a more precise 5 (±0.5) × 10−10
cm3/sec standard He leak revealed the total helium
liberated during these initial measurements was
slightly underestimated. The general procedure was
to measure helium evolution from a group of
zircons at progressively higher temperatures of
400°C, 600°C, and 1000°C for 20 sec intervals.
(Previous studies of helium diffusion (Magomedov,
1970) from zircons indicated 1000°C was sufficient
to liberate the helium with an activation energy
of 15 kcal/mol.) We did not include the small
amount of He observed at 1100°C in the total He
summation because of possible atmospheric
contamination. Between six and eight groups of
zircons were analyzed at each depth. Runs were
repeated at a given temperature until background
helium levels were observed. Data recordings and
integration under the peaks were done with a
Nicolet 1170 signal averager.

The third column in Table 1 shows, as a function
of depth, the total amount of He liberated
per μg of zircon for zircon groups comprised of
approximately equal-size (~50-75 μm) zircons. The
fourth column in Table 1 shows the ratio of the
amount of He actually measured in zircons from any
particular depth to the estimated amount of He
which should have accumulated in those same
zircons assuming negligible diffusion loss. For
the zircons taken from a surface outcrop we assumed
this ratio was one because the specimens we
used were small fragments from the interior of
larger zircon crystals.

For the other zircons from the granite and
gneiss cores, we made the assumption that the
radiogenic Pb concentration in zircons from all
depths was, on the average, the same as that
measured (Zartman, 1979) at 2900 m, i.e., ~80 ppm
with 206Pb/207Pb and 206Pb/208Pb ratios of ten
(Gentry, et al., 1982; Zartman, 1979). Since every
U and Th derived atom of 206Pb, 207Pb, and 208Pb
represents 8, 7, and 6 α-decays respectively, this
means there should be ~7.7 atoms of He generated
for every Pb atom in these zircons.

Knowledge of the zircon mass and the appropriate
compensation factor (to account for differences
in initial He loss via near-surface α-emission) enabled us to calculate the theoretical
amount of He which could have accumulated assuming
negligible diffusion loss. This compensating factor
is necessary because the larger (150-250 μm)
zircons lost a smaller proportion of the total He
generated within the crystal via near-surface α-emission than did the smaller (40-50 μm) zircons.
For the smaller zircons we estimate as many as 30-40% of the α-particles (He) emitted within the
crystal could have escaped initially whereas for
the larger zircons we studied only 5-10% of the
total He would have been lost via this
mechanism. The ratio of the measured to the
theoretical amount of He is shown in the last
column of Table 1. The uncertainties in our
estimates of the zircon masses and compensation
factors probably mean these last values are good
only to ±30%.

Table 1: The values listed below show first,
as a function of depth and temperature, the
amount of helium liberated from various groups
of zircons in units of 10−8 cc per μg and
second, the ratio of the amount of helium
liberated to the theoretical amount which would
have been retained assuming no diffusion loss.
The near equality of the He concentrations in
the surface and 960 m depth zircons is not
particularly meaningful because the surface
zircons were from an entirely different
geological unit and doubtless have different U-Th-Pb concentrations than the zircons from the
core samples.

SampleDepth (m)

SampleTemp. (°C)

He(10−8 cc/μg)

He(measured)He(theoret.)

Surface

20

8.2

1

960

105

8.6

0.58

2170

151

3.6

0.27

2900

197

2.8

0.17

3502

239

7.6 × 10−2

1.2 × 10−2

3930

277

~2 × 10−2

~10−3

4310

313

~2 × 10−2

~10−3

In spite of these uncertainties, it is quite
evident from Table 1 that the zircons from 960 m
seem to have retained considerable amounts of
He, and perhaps more significantly, differential
He loss with increasing depth (and temperature)
has occurred rather slowly down to 2900 m
(197°C) before a precipitous drop is observed at
239°C (3502 m). In fact, at present we are not
certain whether the minute amounts of He
recorded from the deepest zircons (3930 and 4310
m) are actually residual He in the zircons or
derived from some other source. That is, in the
two deepest zircon groups (3930 and 4310 m), we
observed only short bursts of He (~l-2 sec) in
contrast to the prolonged 20 sec or more
evolution of He which was typical of He
liberation from zircon groups down to and
including 3502 m. In fact, it was this prolonged
He liberation profile seen in two 150-250 μm
size zircon groups from 3502 m which convinces
us that some residual He is still trapped in the
zircons down to that depth (239°C).

Now it was recently noted that the high
retention of Pb in even the deepest granite
cores had favorable implications for nuclear
waste containment in deep (1000 to 3000 m)
granite holes (Gentry, et al., 1982). The
rationale for these implications is
straightforward: If zircons, which have been
exposed to the same type of elevated temperature
environment anticipated in deep granite burial,
show no detectable Pb loss either from higher
temperatures or from aqueous solution corrosion
effects, then nuclear wastes buried in that same
granite should, if anything, experience even
greater retention because of the comparative
immobility of waste-type elements as compared to
Pb.

The present results are important in that
they provide clear evidence that the dominant
factor in slow He loss down to 2900 m is attributable
to greater diffusion loss at higher
temperatures rather than any corrosion induced
losses from the zircons. This is not at all
surprising because microscopic examination shows
first that zircons from all depths exhibit well-defined prismatic faces without any evidence of
external corrosion, and secondly that the
delicate internal inclusions within the zircons
do not show any evidence of alteration from
aqueous intrusion via any microstructural
defects. Indeed, the relatively slow liberation
of He over several 20 sec intervals observed in
zircons from the surface all the way down to
2900 m is strong evidence that these zircons are
virtually free of any microfractures which would
have permitted a more rapid He escape. In fact,
considering the Precambrian age of the granite
cores (Zartman, 1979), our results show an
almost phenomenal amount of He has been retained
at higher temperatures, and the reason for this
certainly needs further investigation for it may
well turn out to have a critical bearing on the
waste storage problem.

Thus the additional evidences reported herein
considerably reinforce the view that deep-granite
storage should be a very safe corrosion-resistant waste containment procedure. The
certainty of these results stands in clear
contrast with the uncertainties about how well
alternative storage sites (e.g., salt domes)
could withstand corrosion and/or dissolution
from intruding aqueous solutions.

Acknowledgments. This research was sponsored
by the U. S. Department of Energy, Division of
Basic Energy Sciences, under contract W-71105-eng-26 with Union Carbide Corporation. We thank
A. W. Laughlin of the Los Alamos National Laboratory
for providing the core samples.